1. Field of the Invention
The present invention relates to a two component developer composition useful in magnetic brush development which exhibits high performance even under low-temperature and low-humidity conditions and under high-temperature and high-humidity conditions.
2. Description of the Related Art
The electrostatic chargeability of a toner is an important factor on the quality of the resulting print. However, this electrostatic chargeability varies depending upon the environmental temperature and humidity, which causes a problem in that the quality of a print is so affected by environmental factors as to be deteriorated under low-temperature and low-humidity conditions owing to an increase of the electric charge of the toner or under high-temperature and high-humidity conditions owing to a decrease thereof.
The binder resin used in the preparation of a toner includes polystyrene; styrenic copolymers, such as styrene-butadiene copolymer and styrene-acrylic copolymer; polyethylene; ethylenic copolymers such as ethylene-vinyl acetate copolymer; poly(meth)acrylate; polyester; epoxy resin; and polyamide resin. In electrophotography, a polyester is widely used as a binder resin for toners by virtue of its excellent fixability and negative chargeability. Further, the superiority of a polyester is widely supported in the field of color toners from the standpoints of not only fixability and negative chargeability, but also transparency. Under these circumstances, it has been expected to develop a toner or a developer composition which exhibits the excellent characteristics inherent in a polyester and is little influenced by environmental factors.
For the purpose of preparing such a toner or a developer composition, there have recently been disclosed processes for modifying the polyester, for example, a process of controlling the acid value of the polyester (process 1), a process of controlling the sum total of the acid value and hydroxyl value of the polyester (process 2), a process of displacing the carboxyl groups of the polyester with nitrogenous functional groups (process 3), a process of surface-treating a toner comprising a polyester having a low acid value and a low hydroxyl value to make it hydrophobic (process 4), and so on.
However, the processes 1 and 2 are disadvantageous in that the resulting toner is poor in fixability because the modified polyester has a decreased content of polar groups, such as carboxyl groups. Further, it has been pointed out with respect to the process 3 that when the resulting toner is negatively charged, an increase of fog and a lowering in the transfer efficiency occur.